This invention relates to cranes, and in particular, to pedestal mounted single boom cranes.
In the prior art, such cranes were normally used to lift five or six ton loads to a height of perhaps 70 feet. These cranes, as shown in FIG. 5 of the drawings, were tubular in section; the tube being made of a steel plate of perhaps 1/4 inch thick, formed with slightly rounded corners and welded at one longitudinal edge as at 10 in FIG. 5. Structural plates were welded along one or more sides as indicated at 12, 14, 16 and 18. These plates extended various distances along the side portions of the tubular crane to compensate for the loads which could be expected. In practice, the length of the single boom is limited and standard sizes are 30, 40 and 50 feet.
A crane such as that shown in FIG. 1, comprises in addition to the boom 20, a mast 22, a pivot pin 24 about which the boom can pivot in a vertical arc for a distance of perhaps 70.degree. (as indicated by the arrow A), and a stick cylinder 26 which is operative to move the boom in this arc. The mast itself can rotate in a horizontal plane about a fixed base. Toward the end of the boom is a pulley (not shown) about which is wound a cable 28 to support the load 30. The pedestal is provided with means for withdrawing and playing out the cable.
Considering the boom under the static load conditions shown in FIG. 1, the load would tend to bend the boom to the position shown in phantom lines. This position is shown exaggerated to illustrate the effects of the load. In addition to the external load shown at 30, the weight of the boom itself will tend to produce this condition.
In designing such boom in accordance with the prior art, material had to be provided to overcome the expected working stress. Not considering factors of safety, this working stress comprises the stress due to a load plus the stress due to a weight of the boom material. It is known in the prior art to make such booms of steel having a high yield stress on the order of 50,000 kips.
Such booms have been limited in length and capacity primarily because the mass of the boom increases tremendously as the length of the boom increases.